The uplink capacity in a code division multiple access (CDMA)-based system, such as high speed packet access (HSPA), or a single channel frequency division multiple access (SC-FDMA) system, such as an evolved universal terrestrial radio access network (E-UTRAN), is limited by interference. For a CDMA-based system, uplink interference at a specific cell site is typically generated by WTRUs, (i.e., users) connected to the cell as well as WTRUs connected to other cells. In the case of an SC-FDMA-based system, uplink interference stems primarily from WTRUs connected to other cells. To maintain coverage and system stability, a cell site can tolerate only up to a certain amount of uplink interference at any given instant in time. As a result, system capacity is maximized if interference can be kept constant as a function of time. This consistency allows a maximum of users to transmit and/or generate interference without having the uplink interference exceeding a predetermined threshold at any time.
High-speed uplink packet access (HSUPA), as defined in the Third Generation Partnership Project (3GPP) Release 6, employs HARQ with synchronous retransmissions. When utilizing a 2 millisecond (ms) transmission timing interval (TTI), the minimum instantaneous data rate is often larger than the data rate offered by an application, due to the need to transmit a number of bits that is at least the size of a single radio link control (RLC) protocol data unit (PDU) in a given TTI. When this occurs, a WTRU can utilize only a subset of the available HARQ processes. As a result, the interference generated by a given active WTRU is not constant over a time span of eight (8) TTIs. During some TTIs, the WTRU transmits data and the interference it generates is high. During other TTIs, the WTRU may only transmit control information and, therefore, the interference it generates is low. In order to equalize interference across all TTIs, the system can restrict each WTRU to use a certain WTRU-specific subset of HARQ processes, and select different subsets for different WTRUs.
Transmissions from a WTRU for a certain stream of data may be managed by non-scheduled transmissions or scheduling grants. With non-scheduled transmissions, the WTRU can freely transmit up to a fixed data rate in certain HARQ processes. With scheduling grants, the WTRU can also transmit up to a certain data rate on certain HARQ processes, but the maximum data rate is subject to change dynamically depending on the maximum power ratio signaled by a Node-B at a given time.
When the network manages the transmission by allowing non-scheduled transmissions, the set of HARQ processes is signaled to the WTRU through radio resource control (RRC) signaling. The Node-B determines the set of HARQ processes and signals this information to the radio network controller (RNC), which then relays it to the user through RRC signaling. An advantage of managing delay-sensitive traffic with non-scheduled transmissions is that it eliminates the possibility of any additional delay that could be caused by insufficiency of the resources granted by the Node-B when managing the transmissions with scheduling grants. Another advantage is that it eliminates the signaling overhead due to the transmission of scheduling information that is required with scheduling grants.
With the currently defined mechanisms for non-scheduled transmissions, however, the performance of the system is sub-optimal when the application mix is dominated by delay-sensitive applications that generate traffic patterns exhibiting periods of high activity alternated with periods of low activity. An example of this type of application is the voice over Internet protocol (VoIP) application, in which silence periods translate into a very low amount of traffic needing to be transmitted. When the cell or system is dominated by this type of application, capacity is maximized only if the network is capable of modifying the subset of HARQ processes used by a WTRU when its activity state changes, so that the interference is always equalized across the HARQ processes. Otherwise, the network has to restrict the number of WTRUs utilizing a certain HARQ process so that the threshold is not exceeded, even when they are all active at the same time, resulting in a much lower capacity.
An issue when utilizing non-scheduled transmissions is that it allows modification of the subset of allowed HARQ processes only through RRC signaling, which typically involves latencies of several hundreds of milliseconds. This latency is significant compared to a typical interval between changes of activity for applications such as voice applications. Furthermore, RRC signaling in the current Release 6 architecture is controlled by the RNC. Therefore, the Node-B needs to signal the modification of the subset of allowed HARQ processes to the RNC beforehand. The interval of time between the change of activity state at the WTRU and the effective change of HARQ processes may well be larger than the duration of the activity state. Accordingly, this becomes unworkable for equalizing interference across HARQ processes.
It would therefore be beneficial to provide a method and apparatus for dynamically allocating HARQ processes in the uplink that would aid in optimizing capacity with non-scheduled transmissions.